The effects of multiple stressors on wetland communities: pesticides, pathogens and competing amphibians JULIA C. BUCK*, ERIN A. SCHEESSELE*, RICK A. RELYEA † AND ANDREW R. BLAUSTEIN* *Zoology Department, Oregon State University, Corvallis, OR, U.S.A. † Department of Biological Sciences, University of Pittsburgh, Pittsburgh, PA, U.S.A. SUMMARY 1. Anthropogenic effects have propelled us into what many have described as the sixth mass extinction, and amphibians are among the most affected groups. The causes of global amphibian population declines and extinctions are varied, complex and context-dependent and may involve multiple stressors. However, experimental studies examining multiple factors contributing to amphibian population declines are rare. 2. Using outdoor mesocosms containing zooplankton, phytoplankton, periphyton and tadpoles, we conducted a 2 · 2 · 3 factorial experiment that examined the separate and combined effects of an insecticide and the fungal pathogen Batrachochytrium dendrobatidis (Bd) on three different assemblages of larval pacific treefrogs (Pseudacris regilla) and Cascades frogs (Rana cascadae). 3. Larval amphibian growth and development were affected by carbaryl and the amphibian assemblage treatment, but only minimally by Bd. Carbaryl delayed metamorphosis in both amphibian species and increased the growth rate of P. regilla. Carbaryl also reduced cladoceran abundance, which, in turn, had positive effects on phytoplankton abundance but no effect on periphyton biomass. Substituting 20 intraspecific competitors with 20 interspecific competitors decreased the larval period but not the growth rate of P. regilla. In contrast, substituting 20 intraspecific competitors with 20 interspecific competitors had no effect on R. cascadae. Results of real-time quantitative polymerase chain reaction (qPCR) analysis confirmed infection of Bd-exposed animals, but exposure to Bd had no effects on either species in univariate analyses, although it had significant or nearly significant effects in several multivariate analyses. In short, we found no interactive effects among the treatments on amphibian growth and development. 4. We encourage future research on the interactive effects of pesticides and pathogens on amphibian communities. Keywords: amphibian population decline, Batrachochytrium dendrobatidis, carbaryl, competition, mesocosm Introduction A current challenge in ecology is to identify multiple and complex causes of species extinctions (May, 2010). At the forefront of the biodiversity crisis are amphibians (Blau- stein & Kiesecker, 2002; Wake & Vredenburg, 2008), with recent estimates suggesting that more than 40% of amphibian species are experiencing population declines and extinctions (Stuart et al., 2004). The causes of global amphibian population declines are varied and complex. Stressors may operate independently of one another or synergistically (Blaustein & Kiesecker, 2002; Hayes et al., 2010; Blaustein et al., 2011). Identifying interactions between multiple stressors through experimental investi- gations may suggest how future declines can be mini- mised. With increasing land-use change, contaminants are a ubiquitous threat in amphibian breeding habitats. One recent survey determined that 30–60% of shallow ground water and 60–95% of streams across different land-use categories in the U.S.A. are contaminated with at least one pesticide (Gilliom, 2007). Contaminants in high doses can have direct lethal effects on amphibians. More often, however, concentrations of contaminants in natural Correspondence: Julia C. Buck, Zoology Department, Oregon State University, 3029 Cordley Hall, Corvallis, OR 97331, U.S.A. E-mail: buckj@science.oregonstate.edu Freshwater Biology (2012) 57, 61–73 doi:10.1111/j.1365-2427.2011.02695.x Ó 2011 Blackwell Publishing Ltd 61